This invention relates to methods and apparatus for regulating flow of gas from one point to another. For example, the methods and apparatus of this invention may be used to regulate flow of gas from a gas container or cylinder to its intended destination in an environmental monitoring equipment. More particularly, but not by way of limitation, this invention relates to a method and apparatus for regulating the quantity and quality of gas flow from a portable gas container to a device that monitors environmental conditions for the purpose of calibrating the environmental monitor prior to use.
Today's complex industrialized world presents high levels of danger and difficulty for workers of all kinds. Among the more dangerous conditions workers face everyday is the risk of encountering hazardous air. Hazards in the air may range from causing discomfort to causing immediate death. In this range is included explosive gases to subtle health hazards which lead to great harm with repeated exposure over a period of time. The three most common hazards are air containing high levels of a) oxygen, b) toxic gases such as carbon monoxide and hydrogen sulfide, and c) combustible gases including vapors commonly existing as liquids.
Oxygen is an odorless, colorless and tasteless gas that supports life but also makes combustion possible. When oxygen deficiency in air is encountered it becomes a hazard in that such deficiency may result in asphyxiation in confined spaces. On the other hand, too much oxygen may result in an explosive atmosphere. Next, toxic gases may result in death after short exposure or in other cases result in harmful physiological changes caused by repeated long-term exposures. Combustible gases ignite resulting in explosions especially where oxygen is present.
Thus dangerous environments containing unsafe quantities of hazardous air must be detected and avoided or ventilated (made safe). Detection of hazardous air is done through gas detectors or monitors of various types. The specification of the present invention will interchangeably use gas detectors or monitors with the intent that they mean the same item. Gas detectors or monitors are commonly used in the following situations: a) any entry into a confined space; b) any "hot-work" spaces such as welding, cutting or using electrical items in and around potentially combustible gases; c) time-weighted average personal exposure monitoring such as exposure to a given toxic gas over some period of time; d) leak detection as in where a known gas is leaking; and e) an emergency response--especially when conditions are unknown. Thus, armed with correct information, any worker may be assisted in approaching an environment with safety.
Many conventional gas detectors employ various sensing technologies to be able to detect the presence of one or more potential gas hazards. The central element to all of the various sensing technologies is that sensor in the gas detector has a known steady state parameter in normal clean air and that parameter changes as it comes into contact with the gas or gases intended to be monitored. It is this change in the parameter from the known starting point that allows the measurement of the various gases in the air.
Current sensor technologies include a) electro-chemical sensors, commonly used for detection of oxygen and toxic gases, b) catalytic beads or pellistors, commonly used for detection of combustible gases, and c) metal oxide sensors, a newer form of sensor used for detection of combustible and toxic gases. Combination of the these three sensing technologies are most commonly found in portable personal safety gas detectors. Other more sophisticated devices include photo-ionization detection and flame ionization detection.
Electro-chemical sensors have a housing containing a specially mixed electrolyte which is intended to react with a gas (or gases) that is (are) intended to be monitored while not reacting with other gases that may also be present in the air. Depending on the type of electrolyte, the sensor may be more or less sensitive unless the monitor is calibrated. The catalytic beads or pellistors actually oxidize or burn the gas as it passes over the electric wire filaments in the sensor. One of the circuits in the sensor is specially treated with various catalysts to allow it to react differently with the burning gas than other untreated wire filaments in the sensor. By measuring the change in the electrical properties between the treated portion of the circuit and the untreated portion of the circuit, the sensor is able to give an indication of gas presence and level. Metal oxide sensors also operate on the principle of changing electrical properties within the circuits due to the exposure to various gases.
As a result of the very sensitive nature of these various sensors, it is extremely important that the gas monitors or detectors be calibrated often to obtain accurate sensor readings. As in the case of periodic tuning of a car, a gas monitor or detector needs tuning on a frequent basis. The checking for accuracy of sensors is accomplished by exposing the sensors to a known level of gas and taking readings; for example, a gas detector containing a combustible sensor, an oxygen sensor and a carbon monoxide sensor is exposed to a premixed gas of various levels of oxygen, carbon monoxide and a known combustible. If after exposure, the readings obtained from the gas detector agree with the predetermined and known levels of the premixed gas, the detector is in calibrated and may be safely used. However, if one or more of the readings varies from the known levels in the premixed gas, the detector must be calibrated according to the calibration instructions and once the reading agrees with the premixed gas, the detector may be safely used. If the sensors do not respond or fail to remain calibrated, such condition provides an indication that there is a fault in the unit or very possibly that the sensor itself has degraded to the point where it must be replaced.
The apparatus used to calibrate gas detectors comprises a calibration kit. It must be understood that to obtain the most accurate calibration of the gas monitor, it is very important that the flow of the sampling gas must be smooth, consistent and most nearly simulate the environmental conditions. The process of calibration is performed in a well ventilated location by slowly proceeding to open the gas container or cylinder by adjusting its regulator valve and allowing flow of the required gas to flow to the gas monitor or detector. After about three or so minutes of gas flow, gas readings are taken and verified that the respective sensor's reading matches with the known gas concentration parameter. Again, it cannot be over-emphasized that the condition of the gas flow determines to a great degree the quality of the calibration of the monitor.
One problem, most often, encountered in this type of calibration process is that the flow of gas to the detector is not well regulated and therefore the readings become unreliable. For example, if the gas flow varies in flow quantity, the sensor reading will accordingly vary simply because the exposure is different at every moment in time as the reading is observed.
As a result, a common industry custom developed to overcome this flow control problem by installing a sampling bag between the regulator valve and the gas monitor. The convention has been to fill up the bag with the calibration gas of known concentration by opening the valve for some time and then adjusting it to closed or nearly closed position as the bag is filled up. The bag is then squeezed and the calibration gas is then pumped into the gas monitor with the help of a aspirator pump located in the gas monitor as needed. Obviously, there is great potential for error and studies have shown that the gas monitor encounters differing readings as the bag deflates.
In addition to the above problem, severe other problems are encountered in using this prior art technology in that the calibration gas is diluted at times and contaminated at other times. Still further, the calibration gas is absorbed and diffused in the bag at other times.
Another disadvantage or severe problem encountered by the conventional method of calibration is that the draw rate of the pump in the gas detector or monitor varies from pump to pump. This variance in pump draw rate cannot be compensated by the current technology since the flow regulator valves operate at preset values. Another related problem is that the gas container or cylinder pressure varies as the quantity of gas decreases. Thus with simple preset flow regulators, as the pressure goes down, the flow condition out of the regulator valve changes resulting in relative flow variances and resulting fluctuating (and unreliable) sensors readings. Similarly, as the battery power supply wears down, pump draw rates also begin to vary and again resulting in unreliable readings. Thus conventional apparatus and method of calibrating gas detectors is cumbersome and possesses many disadvantages as well as dangers. Therefore, there is a desperate need for a new apparatus or device and method which permits gas flow which matches fluctuation and changes in flow while operating in a safe manner.